<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>

    <script>

        // 定义变量
        /*var a = 3;
        alert(a);

        a = "abc";
        alert(a);*/

        // 定义number类型
        var num = 1;
        var num2 = 1.2;
        var num3 = NaN;

        document.write(num + "<br>");
        document.write(num2+ "<br>");
        document.write(num3+ "<br>");

    </script>
</head>
<body>

</body>
</html>